In the ITU-T G.842 standard recommendations, a DNI protection structure is proposed. FIG. 1 shows the basic network topology of the DNI protection structure, wherein, the node A and the node F can be used as the add node and the drop node respectively, the nodes B, C and A are on the same ring, the nodes D, E and F are on the other ring, two rings are interconnected by dual nodes, the nodes B˜E connecting the two rings are known as the interconnection nodes. In the DNI protection structure, the two rings might also have other nodes.
In each transmission direction of the DNI protection structure, the signal is transferred along both sides of the first ring from the add node (also can be called as a source node). When the dual-transferred signals reach an interconnection node, they drop at the node and continue to transfer to another interconnection node. In this way, each interconnection node can choose one of the two signals from different routes on the ring. Subsequently, the output of each interconnection node is transferred to the interconnection nodes on the second ring (also known as the drop ring). Each interconnection node on the second ring obtains and transmits the corresponding signal to the drop node (also called as the destination node or host node). Finally, the drop node selects one of the two signals coming from both directions along the second ring to drop. Due to the symmetry of this structure, two interconnection nodes are completely equivalent. In the context, the first ring where the add node is located is called as the add ring, the second ring where the drop node is located is called as the drop ring.
In the prior art, there is no available service protection method proposed based on the abovementioned DNI protection structure.